The polar loop transmitter was first described by Gosling and Petrovic in Electronics Letters, 1979, 15 (10) pp 286-288. This was a development of the work of Kahn “Single Sideband Transmission by Envelope Elimination and Restoration,” Proc. IRE 1952, 40, pp 803-806. The basic scheme of the polar loop transmitter is shown in FIG. 1.
Referring to FIG. 1, the transmitter 100 generally comprises an RF input 101 to which is applied in use an input signal, and a voltage controlled oscillator (VCO) 102. A signal output of the VCO 102 is fed via a controllable gain amplifier 104 to an RF output 103, to provide a modulated RF output signal. Similarly, the RF output 103 is connected both to a second amplitude detector 107 and to a second limiting amplifier 108. This arrangement, therefore, separates both input signals and output signals into amplitude and phase components.
The outputs of the limiting amplifiers 106, 108 are connected to respective inputs of a phase comparator 109, which generates a signal on its output which is proportional to the difference in phase between respective signals at RF input 101 and RF output 103. The output of the phase comparator 109 is connected to a control input of the VCO 102, via a low-pass filter 110, to control the phase of the signal generated by the VCO in order to minimize the phase difference. This arrangement thus constitutes a phase locked loop.
Instead of being coupled to the output of the amplifier 104, the input of the limiting amplifier 108 may be coupled directly to the output of the VCO 102. This variant is not so beneficial since there is no compensation of amplitude to phase variations introduced in the amplifier 104. Outputs of the amplitude detectors 105 and 107 are connected to respective inputs of a comparator 111, which provides a signal on its output dependent on the difference in the instantaneous amplitudes of the respective signals at RF input 101 and RF output 103. The output of the comparator 111 is connected to a gain control input of the controllable amplifier 104, via a second low-pass filter 112. The controllable gain amplifier 104 is, therefore, caused to modulate the output of the VCO 102 so that its amplitude follows variations in the amplitude of the input signal.
The polar loop transmitter 100 suffers certain disadvantages when the variation in input signal amplitude is large, namely that any phase modulation to amplitude modulation conversion caused by the limiting amplifiers 106 and 108 add unwanted phase modulation to the output signal as the amplitude of the input signal varies. In addition, with large RF input signal variations, the difference between the signals at the inputs of the comparator 111 becomes small when the output signal is small, increasing distortion at low instantaneous signal levels. These problems were discussed by Petrovic and Kaya in their paper “Spurious Emissions from EER Type Transmitters,” ISBN 0903748630, pp 181-8. Furthermore, any distortion introduced by the amplitude detectors 105 and 107 appears in the output signal. To minimize this latter effect, it is desirable to control carefully the amplitudes of the signals provided to the amplitude detectors 105 and 107.